Soft Lewis acid/hard Bronsted base

A ternary soft Lewis-acid/hard Bronsted-base/hard Lewis-base catalytic system for the direct catal5rtic enantioselective addition of allyl cyanide 37 to ketones to give tertiary alcohols with a Z-olefin (38) has been developed by Shibasaki (Scheme 2.24). Mechanistic studies revealed that Cu(i)/chiral phosphine ligand 35 and Li(OC6H4-p-OMe) serve as a soft Lewis acid and a hard Bronsted base, respectively, allowing for deprotonative activation of 37 as the rate-determining step. The hard Lewis base, bis(phosphine oxide) 36,... 

Which catalyst should be chosen for a given reaction will depend upon chemical, steric, and mechanistic factors. The application of Pearson s soft and hard acid-base (SHAB) principle has often proved a valuable qualitative guide as to suitable surface sites for a particular reactant. In fact, certain solids actually owe their catalytic power to attached Bronsted or Lewis acid and base groups as exemplified by weak acid ion exchange resins (Sect. 2.3), alumina (Sect. 3.2), and sometimes charcoals. Steric aspects can be con-... 

See also base Bronsted acid hard acid Lewis ACID pH SCALE SOFT ACID. 

It is difficult to quantify the"hardness"or"softness" of a Lewis acid or base. We cannot measure a pKa value, as is the case when assessing the strength of Bronsted-Lowry acids. 

In some ways, this acid-base terminology is unfortunate, since there are other, more common descriptors (e.g. Bronsted, Lewis, hard and soft). However, the terminology has been helpful in suggesting lines of research for the study of non-aqueous systems, and its use will probably continue. 

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